(1) Field of the Invention
The present invention relates to the use of a plasma sprayed outer layer on top of a thermal barrier coating to block the penetration of molten sands into the thermal barrier coating.
(2) Prior Art
Turbine engine airfoils used in desert environments may degrade due to sand related distress of thermal barrier coatings. The mechanism for such distress is believed to be caused by the penetration of fluid sand deposits into 7YSZ ceramic thermal barrier coatings that leads to spallation and then accelerated oxidation of exposed metal. It has been observed that gadolinia stabilized zirconia coatings react with fluid sand deposits and a reaction product forms that inhibits fluid sand penetration into the coating. The reaction product has been identified as being a silicate oxyapatite/garnet containing primarily gadolinia, calcia, zirconia, and silica.
There remains a need however for a coating system which effectively deals with sand related distress.